Gene Cloning

Glossary 451
Repressor A protein which decreases the level
of expression of a gene or operon by binding
to the upstream region.
Restriction endonuclease A naturally occurring
bacterial enzyme which recognizes and
cuts DNA molecules at specific sites. Type II
restriction endonucleases are used extensively
in gene cloning. Also called a restriction
enzyme.
Restriction fragment A piece of DNA created
by cutting DNA with a restriction endonuclease.
Restriction map A diagram showing the relative
positions of sites for restriction enzymes
on a DNA molecule.
RFLP Restriction fragment length polymorphism.
A difference in the length of a specific
restriction fragment found in different members
of the same species. Usually caused by a
small mutation which creates or destroys a
restriction endonuclease recognition site, or
by a variation in the number of repeats of a
tandemly repeated sequence.
RNA interference/RNAi Reduction of expression
of a particular gene by expression of
double-stranded RNAs complementary to
part of the gene.
Segregation The separation of chromatids
(identical replicated copies of chromosomes)
in mitosis or meiosis.
Shine–Dalgarno sequence Another name for
the ribosome binding site on bacterial
mRNA, named after the people who first
described it.
Signal sequence/signal peptide A sequence of
15–30 amino acids at the N terminus of a
secreted protein which is required for transport
through a membrane and is cleaved off
after secretion.
Start codon The codon (usually AUG) in an
mRNA molecule that codes for the first
amino acid of the protein.
Stop codon A codon in mRNA that does not
encode an amino acid but signals the end of
protein translation. Also called a termination
codon. The codons UGA, UAG and UAA are
the three stop codons in nearly all organisms.
Suicide plasmid A plasmid which can only
replicate in some cell types. If it is introduced
into an incompatible host it must integrate
into the chromosome or be lost as the cell
divides.
Supercoil Double-stranded circular DNA in
which the molecule is twisted on itself as a
result of over- or under-winding.
Telomere A region of repetitive sequences at
the ends of the eukaryotic chromosome,
important in replication and stability.
Template The name given to the DNA molecule
which is copied by DNA or RNA polymerase.
Terminator A DNA sequence that causes the
dissociation of RNA polymerase from DNA
and hence terminates transcription of DNA
into mRNA.
Trait
An inherited characteristic.
Transcription The process by which DNA is
copied into complementary mRNA prior to
protein synthesis.
Transcriptomics The study of actively transcribed
genes in a particular cell or tissue at a
particular time by studying the mRNA content,
usually detecting the expression of specific
genes using a micro-array.
Transformation Alteration in the genetic
make-up of a cell by introduction of exogenous
genetic material.
Transgene A gene from one organism introduced
into another.
Transgenic An organism containing genes
from another organism.
Translation The process by which mRNA is
decoded by ribosomes and proteins are synthesized.
Transposon A mobile genetic element capable
of excision from one location and integration
at another location.